Method for producing pH probes

ABSTRACT

A method for producing a pH measuring probe includes: providing an electrode wire protruding out of both sides of a recording device, the electrode wire being fixed to the recording device with the first end thereof, placing a glass tube over a second end of the electrode wire until reaching a cavity of the recording device, the glass tube and the recording device being fixed together; providing another electrode wire protruding from both sides of a base plate including a recess for the recording device, the other electrode wire being fixed to the base plate with its end protruding; providing a casing comprising first and second openings, the casing and base plate being sealed together, forming the housing; and guiding the glass tube through the recess in the base plate until it protrudes out of an opening in the casing and the recording device is inserted into the base plate.

FIELD OF THE INVENTION

The present invention relates to a method for producing a pH probe.

BACKGROUND OF THE INVENTION

Such probes may be, for example, pH measuring probes for measuring thepH of a liquid or food, such as meat, and they may be installed inportable measuring devices. In the simplest embodiment, such measuringprobes have two electrodes in a housing. A chamber holding a secondelectrode formed by a polymer protolyte gel, for example, is usuallyprovided between the first electrode and the housing.

In manufacturing such measuring probes, it is important for the innerelectrode to have a high resistance in comparison with the outerelectrode, and the amount of measuring liquid exchange between theliquid in the first electrode and the liquid in the second electrodeshould be minimized. Essentially two different designs are known forsuch polymer electrolyte measuring probes. First, there are measuringprobes made completely of glass, and second, there are also measuringprobes made of plastic but with the inner electrode situated in a glasstube. The design and functioning of this electrode are explained brieflybelow.

In the manufacture of glass electrodes, two electrode chambers are firstproduced by glass blowing, with the inner electrode being sealed by a pHglass diaphragm and the outer electrode being situated in an electrodechamber which is fused onto the closed end of the inner electrode. Thisresults in a kind of double-walled glass beaker design.

A liquid electrolyte is cast into the chamber of the inner electrode.The inner chamber is sealed using a foam cylinder which seals the innerchamber like a plug, and a silver wire is passed through the foamcylinder until reaching the electrode bottom. To further seal the innerchamber, silicone is extruded into the rear area of the glass tube.Measuring probes preassembled in this way must then cure for a couple ofhours to secure the silver wire in the inner chamber.

The silver wire of the inner electrode is then connected to a coaxialline; for shielding reasons, it is important for the soldered end of thesilver wire together with the inner insulation of the coaxial line to beimmersed in the glass tube of the inner electrode. Because of the smalldiameter of the glass tube, soldering cannot be performed inside theglass tube, so a coil in the form of a mechanical spring or spiral musttypically be coiled at the end of the silver wire protruding out of theglass tube. After soldering, this coil, which forms the end of thesilver wire, is compressed by the insulation on insertion of insulation.

The chamber of the outer electrode is closed using foam and sealed by asilicone material in the same way. A plastic cap is pushed onto the endof the probe for tensile strain relief of the electrode rod and the capis cast there using an adhesive, typically a two-component epoxy resin.A polymer electrolyte is added to the outer chamber of the measuringprobe under a vacuum.

Such glass electrodes are extremely expensive to manufacture because ofthe multitude of different manufacturing steps. Another problem is that,because of the material of the glass electrode and the small amount ofspace available, a great many complicated manufacturing steps arenecessary during assembly of the glass electrode, hardly allowingadequate yield in automated production. The use of glass tubes inparticular is problematical here because the various components arefrequently assembled inside the fragile glass electrodes or at least intheir immediate proximity. In most cases, this prevents the use ofmanufacturing machines for automation of the manufacturing process.However, at the same time, this also means that because of the pluralityof different manufacturing steps and the need for performing themmanually for the most part, the corresponding glass measuring probes arevery expensive to manufacture.

Meanwhile, there is thus a demand for providing measuring probes formeasuring instruments which are simpler and thus less expensive tomanufacture without restricting their functionality.

Plastic measuring probes are far more easily manufactured than themeasuring probes made of glass as described above. The design of onesuch plastic measuring probe is described in DE 100 04 583 C2, forexample.

The manufacturing steps required for producing such plastic probes areessentially the same as those for producing a measuring probe of glass.Some of the manufacturing steps may be simplified by automation becausefirst of all, the glass sheathing of the electrode need no longer beproduced by glass blowing, which is very expensive. Nevertheless it isalso necessary here to perform a number of manufacturing steps whichhave the unwanted effect of making the plastic measuring probe moreexpensive.

As already described in DE 100 04 583 C2, plastic probes are muchsturdier than glass probes, but they are very sensitive to impact, inparticular in the axial direction. In addition, it is necessary from aneconomic standpoint in particular to occasionally refill or replace theelectrolyte liquid inside the measuring probe. However, in the case ofmeasuring probes made of plastic, this is possible only to anunsatisfactory extent or not at all. In addition, glass measuring probesare characterized in comparison with plastic measuring probes in thatthey may be used even when high hygienic demands must be met or when themedium to be measured has a very high temperature, for example. In somecases, glass probes are much better than plastic measuring probesbecause of the low outgassing of impurities and because of their highthermal stability.

Accordingly, it is desirable to manufacture high-quality measuringprobes without using gluing or casting methods, if possible. Further, itis desirable to provide the simplest possible method for manufacturinghigh-quality probes that is suitable for automation. Yet further, it isdesirable to create a method for manufacturing a measuring probe whichmakes it possible to open the measuring probe again after beingmanufactured.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a method formanufacturing a measuring probe, in particular a pH measuring probehaving a housing and two electrodes, by the following method steps isprovided:

-   (a) in order to form a receptacle device, an electrode wire sheathed    with extruded plastic protruding out of the receptacle device on    both ends is provided; the electrode wire is secured at its first    end on the receptacle device; in order to form the first electrode,    a glass tube is pushed over a second end of the electrode wire until    the glass tube is in contact with a recess in the receptacle device;    the glass tube and the receptacle device are joined together;-   (b) in order to form a base plate having a recess in the form of the    receptacle device, another electrode wire sheathed with extruded    plastic protruding out of the base plate on both ends is provided;    the additional electrode wire is secured on the base plate at its    end protruding out of the base plate;-   (c) a sheathing having a first opening in the form of the base plate    and a second opening in the form of the glass tube is provided; the    sheathing and the base plate are joined to form the housing;-   (d) the glass tube is passed through the recess in the base plate    until the glass tube protrudes out of an opening in the sheathing    and the receptacle device comes in contact with the base plate.

The idea on which the present invention is based is to run the silverwire for the inner electrode directly, i.e., without additionalsoldering and without providing an additional line, from the interior ofthe glass tube to the exterior and to secure it there. The silver wireis then prepared to form the outside contact of the measuring electrode.The particular advantage of the method according to this embodiment ofthe present invention is that it is no longer necessary to performcomplex soldering of a line introduced from the outside or to twist thesilver wire. Each electrode chamber is advantageously manufacturedseparately. Furthermore, the method according to this embodiment of thepresent invention makes a very high quality contact available whichrequires fewer processing steps than known manufacturing methods.

For final assembly, one electrode is simply inserted into the other, andthe space is filled with an electrolyte.

Other advantageous embodiments and refinements of the present inventionare evident from the subclaims and the description with reference to thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in greater detail below on the basisof the exemplary embodiments illustrated in the drawing.

FIG. 1 shows a schematic cross-sectional diagram of a first measuringmodule;

FIG. 2 shows a first method according to the present invention formanufacturing a measuring module according to FIG. 1 on the basis ofsubfigures (1)-(10);

FIG. 3 shows a schematic cross-sectional diagram of a second measuringmodule;

FIG. 4 shows a second method according to the present invention formanufacturing a measuring module according to FIG. 3 on the basis ofsubfigures (1)-(4).

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In all the figures, the same elements or elements having the samefunction are labeled with the same reference notation unless otherwiseindicated.

FIG. 1 shows a measuring device 1 in a schematic cross-sectionaldiagram. Measuring device 1 is designed here as a pH measuring modulefor measuring the pH of liquids, foods, wastewater and the like.Measuring module 1 may be part of a measuring instrument not shown inFIG. 1 or it may be the measuring instrument itself. Measuring module 1has an elongated first electrode 2 and a housing 3 which at leastpartially surrounds first electrode 2. A measuring tip 4 of the firstelectrode protrudes out of an opening 5 provided specifically for it onupper end 6 of housing 3.

Housing 3 is composed of a sheathing 11 and a base plate (carrier plate)12. Sheathing 11 and base plate 12 are preferably made of a somewhatelastic plastic. Housing 3 is tightly sealed to the outside except foran inlet (not shown) for the measuring liquid and opening 5.Additionally or alternatively, housing 3 may also have a safety jacket(not shown) on its upper end 6, protecting measuring module 1 toward theoutside, e.g., from mechanical stress, moisture or the like.

First electrode 2 contains a silver wire 7, which is partially goldplated in an advantageous embodiment and has a sheathing of glassforming a glass tube 8 surrounding silver wire 7 at least in the area ofmeasuring tip 4. Glass tube 8 holds a conventional electrolyte fluid andis sealed to the outside. At its end 13 facing base plate 12, firstelectrode 2 is fixedly joined to base plate 12, e.g., by gluing, whileits other end protrudes out of sheathing 11 in the area of measuring tip4.

The areas between first electrode 2 and housing 3 define a chamber 14.Measuring module 1 also has a second electrode 15 designed as a silverelectrode attached to base plate 12 and protruding into chamber 14.Chamber 14 is advantageously filled with a polymer protolyte solution.First electrode 2 thus forms the inner electrode, with second electrode15 forming the outer electrode. Silver wire 7 of inner electrode 2 isthus situated in an inner chamber 16, with outer electrode 15 beingsituated in an outer chamber 14.

Base plate 12 has a recess 17 which accommodates first electrode 2. End13 of first electrode 2 fits snugly into this recess 17 and is securedthere by suitable means, e.g., sealing rings, locking devices,adhesives, etc.

Base plate 12 also has electric contact faces 18, 19 on its outside.First and second electrodes 2, 15 are electrically connected to thesecontacts 18, 19.

In addition, housing 3 has protective webs 20 for protecting measuringtip 4 on its upper end 6. Measuring tip 4 may be rounded for thisreason. Measuring module 1 here is particularly suitable formeasurements in liquids and therefore is advantageously used inlaboratory measuring instruments.

In addition, FIG. 1 shows a tube 50 which extends to a measuring tip 4and is fixedly connected at one end to base plate 12, protruding out ofhousing 3 in area 6 at the other end. This pointed tube 50 is typicallymade of stainless steel and has a temperature sensor for determining thetemperature of the material to be measured.

A measuring module 1 according to FIG. 1 functions as follows.

To perform a measurement, measuring probe 1 is introduced into amaterial to be measured (not shown). Liquid of the material to bemeasured enters into the area between first electrode 2 and secondelectrode 15 using suitable devices (not shown here), e.g., a diaphragmin the opening area of measuring module 1. Depending on the pH of thematerial to be measured, a potential gradient that develops betweenelectrodes 2 and 15 may be tapped at contacts 18, 19. The voltage thustapped is a measure of the pH.

An advantageous method for manufacturing pH measuring module 1 accordingto FIG. 1 is described below with reference to subfigures 1 through 10of FIG. 2. The numbering used below corresponds to the subfigures inFIG. 2:

-   -   1. A silver wire 7, gold plated in a rear area 7′, is provided.        Gold-plated area 7′ of silver wire 7 is bent at an angle.        Partially gold-plated silver wire 7 is placed in a mold (not        shown) with the gold-plated side facing outward and a plastic        material is extruded around it, thus forming receptacle device        23.    -   2. Receptacle device 23 is then placed in another mold to        produce seals 41, 42, 43. To this end, receptacle device 23 has        grooves 40 on its outside surface. An elastomer is extruded into        these grooves 40 to form sealing projections 41. Receptacle        device 23 also has a central recess 26 out of which silver wire        7 protrudes. In this area of recess 26 and at the upper end of        recess 26, an elastomer is also extruded to form a rubber buffer        42 and an inside gasket 43.    -   3. On its end opposite recess 26, receptacle device 23 also has        a groove 44 running on the outside surface of receptacle device        23. In this area of receptacle device 23, a transverse through        bore 45 is also provided. To attach silver wire 7, it is pushed        with its area 7′ through transverse through bore 45 and pulled        tightly, so that wire 7′ is firmly in contact with groove 44.    -   4. A protruding end of silver wire 7 which protrudes out of        transverse through bore 45 is cut off. Area 7″ of silver wire 7        protruding out of recess 26 is chlorinated, preferably for 20        minutes.    -   5. A glass tube 8 is provided, forming first electrode 2. First        an electrolyte liquid is placed in glass tube 8. Then glass tube        8 with its end open at one side is pushed into recess 26 of        receptacle device 23. Glass tube 8 and receptacle device 23 are        secured to one another through inside gasket 43 and rubber        buffer 42 and the entire system is sealed to the outside. Silver        wire 7 is pushed into inner chamber 16 of glass tube 8 with the        electrolyte fluid contained therein.    -   6. Silver wire 15 of second electrode 15 and a stainless steel        tube 50 for the temperature sensor are placed in another mold        and a plastic is suitably extruded around them. This forms base        plate 12. After extrusion, silver rod 15 and stainless steel        pipe 50 protrude out of base plate 12. Then silver wire 15 with        its end 15′ protruding out of base plate 12 is chlorinated.        -   Stainless steel tube 50 is additionally or alternatively            coated with a plastic 51 on its exterior surface.    -   7. In order to form the temperature sensor, a conventional heat        transfer compound is placed into a tip of stainless steel tube        50. Then a twin-cable NTC wire 52 (negative temperature        coefficient) is inserted into the interior of stainless steel        tube 50. The two ends of NTC wire 52 protrude out of tube 50 on        the side of base plate 12. In the area of base plate 12, a        contact plate 52 is produced by extrusion. Base plate 12        therefore advantageously has a recess 54 into which contact        plate 53 is tightly inserted or engaged. Contact plate 52 has        two contact rods 55 which pass through contact plate 53 and are        soldered to NTC wire ends 52 protruding out of it in an area        facing stainless steel tube 50.        -   Silver wire 15 of second electrode 15 is threaded at its one            end which protrudes out of base plate 12 into a receptacle            device 53, e.g., an eye, provided specifically for this            purpose in base plate 12. Silver wire 15 is thus secured            there tightly. In addition, silver wire 15 forms a contact            face 19 there. The protruding end of silver wire 15 is again            cut off.    -   8. Sheathing 11 of measuring probe 1 is manufactured by        extrusion in a suitably designed mold. Then base plate 12 along        with the temperature sensor and second electrode 15 is pushed        into interior 14 of sheathing 11. The end of sheathing 11, which        is open toward the outside, is then welded to the corresponding        areas of base plate 12, e.g., by ultrasound, thereby sealing it.        A double weld 57, 58 which is advantageously provided here        should prevent the polymer protolyte liquid from subsequently        flowing out. Enlarged subfigures (8 a)-(8 c) show a        corresponding welding method for producing the two welds 57, 58        in three steps.        -   Alternatively it would of course also be possible to use            other joining methods—such as a latching joint, a thread or            the like, although ultrasonic welding combined with a double            weld 57, 58 (see (8 a)-(8 c)) is a particularly preferred            method.    -   9. Base plate 12 has an opening 17 through which first electrode        2 is insertable and receptacle device 23 may be inserted in a        form-fitting manner. Electrode 2 together with receptacle device        23 is pushed through this opening 17 until first electrode 2        protrudes out of opening 5 of sheathing 11 on the other end of        housing 3. Sealing projections 41 arranged in outside grooves 40        of receptacle device 23 ensure that first electrode 2 and/or        receptacle device 23 will be sealed in recess 17 in base plate        12 provided for this purpose.    -   10. After inserting first electrode 2 together with base plate        12, a groove 60 is formed between receptacle device 23 and base        plate 12 in an outer area of recess 17. First an O-ring 61 is        inserted into this groove 60 for sealing purposes. Then a        fixation/adjusting screw 62 is screwed into a thread provided        for this purpose in base plate 12 or receptacle device 23.        Alternatively, it would also be conceivable to provide a        different type of sealing means, e.g., a latching means, instead        of fixation/adjusting screw 62.    -   11. Finally, the polymer protolyte solution is added to outer        chamber 14 through an opening (not shown in FIG. 2) provided for        this purpose in housing 3 of measuring module 1.

FIG. 3 shows a second example of a measuring module 1 manufacturedaccording to the present invention. In contrast with the exemplaryembodiment in FIG. 1, measuring module 1 in FIG. 3 is characterized by asimpler and more compact design. In the area of base plate 12 thesealing device has a less complex design. Only one locking device isprovided here.

Furthermore a temperature sensor 50 has been omitted.

In addition, housing 3 also does not have any protective webs 20 forprotecting measuring tip 4. Measuring tip 4 here is designed in such away that it tapers to a tip at front end 6 and is thus suitable forinsertion into a solid material to be measured, such as meat.

Measuring module 1 is advantageously equipped with an elastic safetyjacket (not shown here) for reasons of stability, tightness and hygiene.

FIG. 4 shows a second method according to the present invention formanufacturing a measuring module 1 according to FIG. 3 on the basis ofsubfigures (1)-(4). The method for manufacturing measuring module 1corresponds here essentially to the method described with reference toFIG. 2. Therefore only a few method steps have been selected asexemplary in FIG. 4.

Method steps (1)-(4) differ from steps (1)-(5) essentially in thatreceptacle device 23 in FIG. 4 has a much simpler design. In particular,sealing projections 41, rubber buffer 42 and inside gasket 43 areomitted here. Only a simple O-ring 46 is provided in groove 40 here.Receptacle device 23 is therefore designed to be elastic. When glasstube 8 is inserted into recess 26, this results in frictional forceswhich secure glass tube 8 with respect to recess 26.

In contrast to FIG. 2, the manufacture of temperature sensor 50 (steps 6and 7) is omitted here.

In summary, it can be concluded that this novel method permits a muchsimpler method of manufacturing a measuring module 1 in which theprevious problems associated with the risk of breaking glass tube 50during its manufacture have been minimized despite extensive automationof the manufacturing process.

The present invention has been described here on the basis of thepreceding exemplary embodiments to explain the principle of the presentinvention and its practical application in the best possible way, butthe method according to the present invention may of course also besuitably modified.

Accordingly, other embodiments of the invention will be apparent tothose skilled in the art from a consideration of the specification orpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with thetrue scope and spirit of the invention being indicated by the followingclaims.

1. A method for manufacturing a measuring probe comprising a housing andtwo electrodes, including the method steps: (a) forming a receptacledevice, including providing an electrode wire sheathed with extrudedplastic, the wire protruding out of the receptacle device on both ends,wherein the electrode wire is attached to the receptacle device at afirst end of the electrode wire, wherein in order to form the firstelectrode, a glass tube is pushed over a second end of the electrodewire until the glass tube comes in contact with a recess in thereceptacle device, and wherein the glass tube and receptacle device areattached to one another; (b) forming a base plate having a recess in theshape of the receptacle device, including providing an additionalelectrode wire sheathed with extruded plastic, the additional electrodewire protruding out of the base plate on both ends, wherein theadditional electrode wire is attached to the base plate at an end of theadditional electrode wire which protrudes out of the base plate; (c)providing a sheathing having a first opening in the form of the baseplate and a second opening in the shape of the glass tube is provided,wherein in order to form the housing, the sheathing and base plate aresealed together; and (d) pushing the glass tube through the recess inthe base plate until the glass tube protrudes out of an opening in thesheathing and the receptacle device comes in contact with the baseplate.
 2. The method as recited in claim 1, wherein to attach theelectrode wire, the electrode wire is pushed by the first end of theelectrode wire through a transverse through bore provided in thereceptacle device, tightened, and an end protruding out of thetransverse through bore is cut off.
 3. The method as recited in claim 1,wherein to attach the electrode wire, the electrode wire is threaded bythe first end into a receptacle means provided in at least one of thebase plate and the receptacle device, and is tightened and a protrudingend is cut off.
 4. The method as recited in claim 1, wherein thesheathing and base plate are sealed by ultrasonic welding.
 5. The methodas recited in claim 4, wherein a double weld is produced.
 6. The methodas recited in claim 1, wherein an electrolyte liquid is added to thehousing after sealing the base plate and the sheathing.
 7. The method asrecited in claim 1, wherein to form the first electrode an electrolyteliquid is added into the glass tube.
 8. The method as recited in claim1, wherein the receptacle device has a groove running on an outsidesurface of the receptacle device on an end of the receptacle deviceopposite the recess, with the electrode wire being placed in the groove.9. The method as recited in claim 1, wherein the receptacle device hasat least one of outside grooves and inside grooves into which anelastomer is extruded for manufacturing rubber projections to seal andsecure the first electrode.
 10. The method as recited in claim 1,wherein the receptacle device has a central recess into which anelastomer is extruded for producing a rubber buffer for buffering andsealing the first electrode.
 11. The method as recited in claim 1,wherein a groove formed between the receptacle device and the base plateis sealed by an O-ring and a screw or a latching means.
 12. The methodas recited in claim 1, wherein to secure at least one of the firstelectrode and the second electrode, the at least one of the firstelectrode and the second electrode is glued or welded onto the housingor extruded with the housing.
 13. The method as recited in claim 1,wherein to form a temperature sensor in the base plate a tube is coveredwith plastic, a heat transfer compound is packed into a tip of the tube,and then a twin cable is inserted into the interior of the tube, withthe two ends of the wire protruding out of the tube being soldered tocontact rods.
 14. The method as recited in claim 13, wherein a recess isprovided in the area of the base plate to accommodate a contact plateand the contact rods are inserted into the contact plate arranged in therecess.
 15. The method as recited in claim 1, wherein parts of theelectrode wire arranged inside the housing are at least partiallychlorinated.
 16. The method as recited in claim 1, wherein parts of theelectrode wire which are accessible from outside the housing are atleast partially gold plated.
 17. The method as recited in claim 1,wherein at least one of the base plate, the sheathing and the receptacledevice is manufactured by inserting into a mold and extruding plasticinto the mold.
 18. A method for manufacturing a measuring probe havingfirst and second electrodes, comprising: providing a receptacle device,wherein a first electrode wire protrudes out of the receptacle deviceand a first end of said first electrode wire is attached to thereceptacle device; positioning a tube over a second end of said firstelectrode wire until the tube comes in contact with a recess in thereceptacle device; providing a base plate having a recess, wherein asecond electrode wire protrudes out of the base plate and a first end ofsaid second electrode wire is attached to the base plate; providing asheathing having a first opening in the shape of said base plate and asecond opening in the shape of said tube; sealing the sheathing and baseplate together to form a housing; and pushing the tube toward the recessin the base plate, wherein the tube protrudes out of said second openingin the sheathing and the receptacle device is in contact with the baseplate, wherein to attach said first electrode wire to said receptacledevice, said first end of said first electrode wire is pushed through atransverse opening provided in the receptacle device, tightened, and aprotruding end is cut off.
 19. The method of claim 18, wherein the tubeis made of glass.
 20. The method of claim 18, wherein the firstelectrode wire and the second electrode wire are sheathed with extrudedplastic.
 21. The method as recited in claim 18, wherein to attach saidfirst electrode wire to said receptacle device, said first electrodewire is threaded by said first end into a receptacle means provided inat least one of the base plate and receptacle device, tightened, and aprotruding end is cut off.
 22. The method as recited in claim 18,wherein an electrolyte liquid is added to the housing after sealing thebase plate and the sheathing.
 23. The method as recited in claim 18,wherein the first electrode is formed by adding an electrolyte liquidinto the tube.
 24. The method as recited in claim 18, wherein thereceptacle device includes a groove disposed on an outside surface ofthe receptacle device at an end opposite the recess, and wherein saidfirst electrode wire is disposed in said groove.
 25. The method asrecited in claim 18, wherein the receptacle device includes grooves intowhich an elastomer is extruded for manufacturing rubber projections toseal and secure the first electrode.
 26. The method as recited in claim18, wherein the receptacle device includes a central recess into whichan elastomer is extruded for producing a rubber buffer for buffering andsealing the first electrode.
 27. The method as recited in claim 18,wherein a groove is formed between the receptacle device and the baseplate and is sealed by at least one of an O-ring, a screw and a latchingmeans.
 28. The method as recited in claim 18, wherein the firstelectrode and the second electrode are glued or welded onto the housingor extruded with the housing.
 29. The method as recited in claim 18,further comprising a temperature sensor disposed in the base plate,wherein said temperature sensor includes a sensor tube covered withplastic, wherein a heat transfer compound is packed into a tip of thesensor tube and a twin cable is inserted into the interior of the sensortube, and wherein two ends of a wire protruding out of the sensor tubeare attached to contact rods.
 30. The method as recited in claim 18,further comprising a contact plate disposed in said recess in the baseplate, and wherein contact rods are inserted into the contact plate. 31.The method as recited in claim 18, wherein parts of the first and secondelectrode wires disposed inside the housing are at least partiallychlorinated.
 32. The method as recited in claim 18, wherein parts of thefirst electrode wire which are accessible from outside the housing areat least partially gold plated.
 33. The method as recited in claim 18,wherein at least one of the base plate, the sheathing and the receptacledevice is manufactured by inserting into a mold and extruding plasticinto the mold.